1. Field of the Invention
The present invention is directed to a method for obtaining an image in a nuclear magnetic resonance tomography apparatus, and in particular to a method in the form of a pulse sequence for rapidly acquiring the data for generating such an image.
2. Description of the Prior Art
Essentially three methods for fast imaging are known in nuclear magnetic resonance tomography, namely the FLASH method, the FISP method and the echo planar method, each with respective versions thereof.
The FLASH method, for example, is described in U.S. Pat. No. 4,707,658. Gradient echoes having small flip angles of the radiofrequency pulse and repetition times that are significantly shorter than the spin grid relaxation times of the nuclear spins under investigation are thereby generated. In this method, the flip angles are selected as the repetition times become shorter, i.e. the faster the pulse sequence becomes. The signal-to-noise ratio also drops with the diminution of the flip angles.
In an especially fast version of the FLASH method, what is referred to as the turbo-FLASH method, with extremely short repetition times, the magnetization is inverted before each measuring sequence in order to prevent the T1 contrast from collapsing. As a result of the necessary spin inversion, however, one must wait for an equilibrium of the spins to be established for every new measuring sequence. A continuous measurement in dynamic equilibrium is therefore not possible.
The FISP method, which is disclosed in detail in U.S. Pat. No. 4,769,603, likewise represents a fast gradient echo method wherein--by contrast to the FLASH method--the phase encoding is reset before every radiofrequency pulse.
In practice, however, a version of the FISP method has hitherto prevailed wherein no complete rephasing of the nuclear spins ensues in at least one spatial direction. In sequences with complete rephasing, which are also referred to as "true FISP", black stripes appear in the image given longer repetition times if especially high magnetic field homogeneity is not present.
What is referred to as the echo planar imaging (EPI) method, as disclosed in European Application 0 076 054, is even faster than the FLASH method or FISP method. At the beginning of the pulse sequence, an RF excitation pulse is emitted to an examination subject under the influence of a slice selection gradient in a first direction. Nuclear spins are thereby excited in a slice of the examination subject. After the excitation, a phase-encoding gradient is activated in a second direction and a readout gradient is activated in a third direction. The first, second and third directions reside perpendicularly to one another. The readout gradient is composed of a pre-phasing pulse as well as of sub-pulses with changing (alternating) polarity. As a result of this changing polarity of the readout gradient, the nuclear spins are dephased and rephased in alternation, so that a sequence of nuclear magnetic resonance signals arises. So many signals are thereby acquired after a single excitation that the entire Fourier k-space is sampled, i.e. the existing information suffices for the reconstruction of a complete tomogram.
The nuclear magnetic resonance signals are phase-encoded, sampled in the time domain, digitized, and the numerical values acquired in this way are entered into a raw data matrix. An image of the examination subject is then reconstructed from this raw data matrix on the basis of a two-dimensional Fourier transformation.
The speed advantage of the EP method is essentially due to the acquisition of a number of signals that is sufficient for the reconstruction of a complete tomogram after a single excitation. All signals, which ultimately represent gradient echoes, must be acquired within the T2* decay. The readout gradient must therefore be very rapidly switched in a bipolar manner, which results in hardware demands which are considerable.
A method wherein the advantages of the echo planar method are combined with the advantages of the EPI method to a certain extent is disclosed in U.S. Pat. No. 5,337,000. Radiofrequency pulses are thereby emitted with a repetition time that is shorter than the spin grid relaxation time. A number of echoes is acquired after every radiofrequency pulse on the basis of a readout gradient that multiply changes in polarity. Differing from the "true FISP" method disclosed in the above-cited U.S. Pat. No. 4,769,603, however, a rephasing of the spins does not ensue in all spatial directions. On the contrary, the slice selection gradient, for example, is not symmetrical relative to the allocated radiofrequency pulse, so that this pulse sequence is not rephased, at least with respect to the slice selection direction.
An article entitled, "3D Steady State Echo Planar Imaging" by A. N. Abduljalil et al., SMRM Abstracts 1994, page 472, describes a similar method for 3D imaging. A complete rephasing in all spatial directions, i.e. a true FISP method, was, however, not applied. Given the repetition times of 54 ms or 108 ms recited therein, the phases of the nuclear magnetic resonance signals from different spatial regions would diverge to such an extent given a true FISP method that a pronounced signal modulation would occur. Stripes in the image that are not tolerable in practice would arise therefrom. This is avoided in the standard FISP method because that gradients are not rephased in at least one direction.